1. Field of the Invention
The present invention relates to dye-donor elements for use in thermal dye sublimation transfer methods, the dye-donor elements comprising a dye/binder layer incorporating a high-molecular dextran derivative binder,which facilitates printing of said dye/binder layer by printing techniques such as a gravure process.
2. Description of the Prior Art
Thermal transfer methods have been developed to make prints from electronic pattern information signals e.g. from pictures that have been generated electronically by means of a colour video camera. To make such prints the electronic picture can be subjected to colour separation with the aid of colour filters. The different colour selections thus obtained can then be converted into electric signals, which can be processed to form cyan, magenta, and yellow electrical signals. The resulting electrical colour signals can then be transmitted to a thermal printer. To make the print a dye-donor element having repeated separate areas of cyan, magenta, and yellow dye is placed in face-to-face contact with a receiving sheet and the resulting sandwich is inserted between a thermal printing head and a platen roller. The thermal printing head, which is provided with a plurality of juxtaposed heat-generating resistors, can selectively supply heat to the back of the dye-donor element. For that purpose it is heated up sequentially in correspondence with the cyan, magenta, and yellow electrical signals, so that dye from the selectively heated regions of the dye-donor element is transferred to the receiver sheet and forms a pattern thereon, the shape and density of which are in accordance with the pattern and intensity of the heat supplied to the dye-donor element.
The dye-donor element usually comprises a very thin support e.g. a polyester support, which is coated on both sides with an adhesive or subbing layer, one adhesive or subbing layer being covered with a slipping layer that provides a lubricated surface against which the thermal printing head can pass without suffering abrasion, the other adhesive layer at the opposite side of the support being covered with a dye/binder layer, which contains the printing dyes in a form that can be released in varying amounts depending on, as mentioned above, how much heat is applied to the dye-donor element.
The dye in the dye/binder layer is usually carried by a binder resin. Known binder resins are cellulose derivatives like ethyl cellulose, hydroxyethyl cellulose, ethylhydroxy cellulose, ethylhydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, cellulose acetate, cellulose acetate formate, cellulose acetate propionate, cellulose acetate butyrate, cellulose acetate pentanoate, cellulose acetate hexanoate, cellulose acetate heptanoate, cellulose acetate benzoate, cellulose acetate hydrogen phthalate, and cellulose triacetate; vinyl-type resins like polyvinyl alcohol, polyvinyl acetate, polyvinyl butyral, polyvinyl pyrrolidone, polyvinyl acetoacetal, and polyacrylamide; polymers and copolymers derived from acrylates and acrylate derivatives, such as polyacrylic acid, polymethyl methacrylate, and styrene-acrylate copolymers; polyester resins; polycarbonates; poly(styrene-co-acrylonitrile); polysulfones; polyphenylene oxide; organosilicones such as polysiloxanes; epoxy resins and natural resins, such as gum arabic.
The dye/binder layer comprising said repeated separate areas of cyan, magenta, and yellow dye carried by a binder may be coated from a solution in appropriate solvents on the subbed support, but the known coating techniques are not quite adapted to the discontinuous repeated coating of three differently coloured dye/binder areas on said very thin support. It is therefore customary, especially in large-scale manufacturing conditions, to print said dye/binder layer on said support by printing techniques such as a gravure process.
However, most binders have one or more disadvantages. For instance, some binders have a low viscosity and thus form a dye/binder composition that does not have an ink-like nature and as a consequence is not printable. Other binders have a suitable viscosity, but are soluble only in solvents such as chlorinated hydrocarbon solvents, which are rejected nowadays from an ecological standpoint. Other binders cause dye crystallization, which is to be avoided since it prevents effective thermal dye transfer and consequently produces low and erratic print densities on the receiver sheet.
Another frequently encountered disadvantage of binders is that when heat is supplied by the thermal printing head to the dye-donor element, the dye/binder layer melts and consequently starts sticking to the receiver sheet. This sticking eventually results in the tearing off of the dye/binder layer.